Southern Rail have provoked fresh criticism from the Rail, Maritime and Transport Union (RMT) after it sent a letter to staff warning of possible sanctions for staff joining industrial action.

RMT hit back at Southern's management team, who they accused of using threatening and intimidating behaviour against staff.

In the latest twist in the long-standing dispute between Southern staff and their employers, RMT members will strike for the 29th time in the year-long row, for a 24-hour period on Wednesday (22 February)

Separately, train drivers union Aslef said on Saturday, it could not rule out the possibility of further strikes by its members after they rejected a deal put forward by the company.

The letter sent by Southern's passenger services director, Angie Doll, is quoted by the Press Association as describing the "immeasurable damage" caused by the dispute. After highlighting recent progress made between the parties, Doll wrote: "It would be a great shame if this progress was lost due to further industrial action."

Striking workers are not eligible for payment of shifts that have not been worked, said Doll, who reminded staff of their contractual obligations to the company and wrote: "taking part in strike action will put you in breach of contract.

She added: "Furthermore we will consider applying additional individual sanctions should RMT call further action in which you participate."

However, RMT leader Mick Cash responded to the letter accusing the company of putting undue pressure on staff, and said the strike would go ahead regardless. He said: "RMT's action remains on and we will not bend to these kind of disgusting dirty tricks from a company that puts private profit before public safety.

"Southern Rail have gone back to a campaign of threats, bullying and intimidation of their staff who are doing nothing more than standing up for passenger safety. It is disgraceful behaviour."

However, the impact of the ongoing dispute extends beyond safety for customers affected by it. Local MPs have spoken of job losses by those commuting in and out of the region; relationships have broken down; so too has the local economy suffered.

According to local Green Party MP Caroline Lucas, local football club Brighton and Hove Albion have lost £300,000 ($372,360) as a result of the dispute. Estate agents have also talked of the risk to house prices with London wages needed in order to afford to buy in the area threatened.

A Southern rail conductor waits as commuters board a train at East Croydon station in LondonJack Taylor/ Getty Images